1. Field of the Invention
This invention relates to a thermoelectrochemical process for production of hydrogen and oxygen from water. The process provides a closed cycle reactant regenerative process for the production of hydrogen and oxygen from water requiring input of water, heat and a relatively small amount of electrical energy. The process is a hybrid copper oxide-copper sulfate water splitting cycle using copper oxide reactant in the reactions producing both hydrogen and oxygen providing two phase reaction products in each step.
2. Description of the Prior Art
Thermochemical cycles to produce hydrogen have been sought in recent years to provide hydrogen for use both as a fuel and as a chemical reactant. Especially in view of ecological and pollution considerations, the use of hydrogen as a fuel has attained greater significance. Hydrogen may be produced from water, a cheap and abundant raw material, and when burned reverts to water, a natural, non-polluting substance. Hydrogen also affords extreme flexibility in its utilization as a fuel providing advantages in many ways over present fuels for power sources such as turbines, reciprocating engines and fuel cells.
The present commercial method for the production of hydrogen is the direct electrolysis of water into its constituent elements of hydrogen and oxygen. The electrolysis process is dependent entirely upon electricity, and therefore its economics are dependent upon generation of electrical energy which is relatively inefficient.
Direct thermodecomposition of water is not promising, since water starts decomposing with a reasonable yield at temperatures of 2,500.degree. to 3,000.degree. C. These temperatures (2,500.degree. to 3,000.degree. C.) are not attainable from heat sources such as nuclear reactors. Therefore, thermochemical cycles for production of hydrogen and oxygen from water at temperatures attainable from the core coolant of nuclear reactors and from solar energy sources would be desirable. With expanded use of nuclear reactors, it would be advantageous to utilize the heat given off by the reactor to produce hydrogen and oxygen from the water molecule thereby providing a new fuel. High temperature gas-cooled reactors have output coolant temperatures in the order of 650.degree. to 950.degree. C. This heat output represents at least 80 percent of the product energy from the nuclear reactor. Recent trends have been toward increases in the output coolant temperature and such temperatures in the order of 1,000.degree. to 1,200.degree. C. can be expected as a development of present technology and as a reasonable goal for nuclear fusion reactors in the future. Thermochemical processes for the production of hydrogen, utilizing such heat energy, have been sought in recent years.
Various thermochemical cycles for the production of hydrogen and oxygen from water have been referred to in U.S. Pat. No. 3,907,980. Other thermochemical cycles for the production of hydrogen and oxygen from water using iron chlorides are disclosed in U.S. Pat. Nos. 3,939,257 and 3,998,942. The process disclosed in U.S. Pat. No. 3,907,980 is illustrative of a process for production of hydrogen and oxygen from water combining thermo and electrochemical reactions. The process disclosed by U.S. Pat. No. 3,907,980 teaches electrically hydrolyzing a cadmium anode and recycling the cadmium to the electrochemical reaction with removal of hydrogen and oxygen from the reaction system.
Thermochemical cycles for the production of hydrogen and oxygen from water utilizing sulfur oxides have been proposed. One process known as The Westinghouse Sulfur Cycle, is a two-step thermochemical cycle for decomposition of water into hydrogen and oxygen wherein oxides of sulfur serve as recycled intermediates within the system. The process is described in the paper The Westinghouse Sulfur Cycle for the Thermochemical Decomposition of Water, L. E. Brecher, S. Spewock and C. J. Warde, in 1st World Hydrogen Energy Conference, Conference Proceedings edited by T. Nejat Veziroglu, Vol 1, Miami Beach, Fla., March 1976. One disadvantage of the Westinghouse Sulfur Cycle is that it involves concentrated acids, principally sulfuric acid, leading to corrosion of process plant components.
Another proposed thermochemical water splitting process for the production of hydrogen and oxygen is a cycle based upon sulfur and iodine which was described first by Russell of General Atomic Company in "Water-Splitting--A Progress Report", First World Hydrogen Energy Conference Proceedings, Vol. 1, Miami Beach, Fla., March 1976. The process is further described in a paper presented at the Ninth Synthetic Pipeline Gas Symposium, Oct. 31-Nov. 2, 1977, Des Plaines, Ill., J. R. Schuster, J. L. Russell, Jr., J. H. Norman, T. Ohno and P. W. Trester, "Status of Thermochemical Water-Splitting Development at General Atomic", General Atomic Report GA-A14666, October, 1977. The General Atomic process involves both sulfuric acid and hydrogen iodide causing corrosion problems in conventional plant process equipment.
U.S. Pat. No. 4,192,726 discloses a thermoelectrochemical reactant regenerative closed cycle process for production of hydrogen and oxygen from water wherein sulfur trioxide is catalytically thermally decomposed in a gas-phase reaction. The reaction products are gaseous and require separation.